1. Field of the Invention
The present invention relates to an apparatus for microbiologically degrading a pollutant, a process for microbiologically purifying a medium containing a pollutant and a process for microbiologically degrading a pollutant.
2. Related Background Art
Recently, there has been a serious problem of environmental pollution caused by hydrocarbons such as aromatic hydrocarbons, paraffin and naphthene, or organochlorine compounds such as trichloroethylene, tetrachloroethylene and trichloroethane.
Many of these compounds penetrate into soils, not degraded, and gradually dissolve in ground water and the contamination spreads through the ground water.
It has been strongly desired to establish technology for preventing such serious environmental pollution from occurring, for remedying the environment which has already been contaminated, that is, restoring the environment to the original state.
Examples of the environmental restoration technology include aeration treatment where the contaminated ground water is pumped up, and volatile organic compounds are separated from the water and adsorbed by activated charcoal, heat treatment where the contaminated soil is exposed to the sun or a heat source to evaporate volatile organic substances with heat, vacuum extraction process in which a borehole was made into the contaminated soil, and the pollutant is sucked under vacuum, or vacuum pan treatment in which the contaminated soil is put in a vacuum pan and heated and sucked to extract the pollutant.
These physical and chemical treatments may be particularly effective for local contamination of high concentration, but not for large region contamination of low concentration in view of processing speed improvement or the cost reduction.
Besides, these organic substances recovered by the activated charcoal adsorption still need further treatment to be made into harmless substances since these organic substances are usually hard to be degraded.
Recently, biotreatment utilizing microorganisms has been studied as soil remediation technology to overcome the aforementioned problems with these physical and chemical treatments.
With the process utilizing microorganisms to degrade the pollutant, particularly microorganisms living in soil, it is expected that the remediation can be carried out by natural energy and with less energy input, and that the degradation proceeds to the final products of water and carbon dioxide.
Under such expectation, bioremediation has recently been attracting attention.
There are many microorganisms known to degrade compounds such as aromatic hydrocarbons and organochlorine compounds which are difficult to be degraded and pollutants of soil and ground water. Also almost all substances required by the microorganisms are known. These substances are gas, liquid or solid. But, when these substances are carelessly fed to the polluted environment, the supply may be excess or insufficient failing to provide balanced supply, they may affect the various ecosystems in the environment or spoil proper bioremediation. In addition, it is required a large scale facility to externally control the accurate supply of these substances into the soil, and the primary object of the bioremediation to exploit natural power as much as possible might be lost.
Conventional methods of bioremediation are classified as follows according to the process of introducing the degrading microorganisms.
The most primitive method is called biofarming, in which the contaminated soil is harrowed to be oxygenated, thereby the microorganisms in the soil are activated and degrade the pollutant. This method stands on the fact that "almost every substance in the soil has its microorganisms for degradation" and the microorganisms are utilized without being particularly specified. This method is effective when the pollutant is easily degradable by microorganisms and present in relatively shallow soil, near the surface. For example, it can be effectively applied to the pollution with hydrophobic hydrocarbons having a relatively small specific gravity or vegetable oils.
"Biostimulation" also utilizes indigenous microorganisms, but nutrients and oxygen are supplied to the contaminated soil to activate the indigenous microbes living in the contaminated for pollutant degradation. For example, this process is often used in purification of the soil contaminated with petroleum. In this case, nutrients other than the carbon source are introduced from outside to maintain the growth and the activity of the microbes to degrade the pollutant, i.e. petroleum.
Accordingly, the technical problem of such remediation methods is the supply of oxygen and nutrients containing nitrogen and phosphorus which are in short. As a similar method employing degrading microbes, there is activated sludge process used in waste water treatment. Activated sludge process, though not a soil remediation process, is similar to the above mentioned process in technology. In the waste water treatment, waste water is usually rich in nutrients for the microorganisms, not only the carbon source but also nitrogen and phosphorus compounds. Thus, oxygen supply is critical for the purification. In those methods, the microorganisms indigenous to the environment to be remedied are activated, and the microorganisms are often not specified.
On the other hand, volatile organochlorine compounds which are highly penetrable into the ground and hard to be degraded may not be easily degraded by the indigenous microbes. In other words, sometimes there are no degrading microbes present since these compounds are artificial substances not present in nature. Even if degrading microbes are present for such compounds, the location deep under the ground is not suitable for the growth of such microbes. There comes the bioaugmentation method characterized in that a microorganism selected for an ability to degrade the hardly-degradable substance is cultured and supplied to the contaminated region together with the functional materials necessary for growth and degradation.
In any of the processes, the common technical problem is supply of nutrients and oxygen to the contaminated region remains the same. Usually air is sent to supply oxygen, and the nutrients is supplied as an aqueous solution.
Since the microbes living in the soil are adhering to a solid or suspended in a liquid, not all of the supplied liquid nutrients or gaseous oxygen are utilized by the pollutant-degrading microorganisms, and a part of the nutrients and oxygen may be used by other microorganisms which do not contribute to degradation of the pollutant, be eluted from the region, or be left unused resulting in a decrease of utilization efficiency.
Consumption of the supplied nutrients by the microorganisms which do not contribute to the degradation of the pollutant not only means that the loss of the nutrients, but also means more oxygen consumption by the activated microorganisms which do not contribute to the degradation preventing the activity of degrading microorganisms. The gas supply has the same problem. In addition, when the nutrients and oxygen which can be easily utilized by the microorganisms are supplied at one time, drastic growth and activation of the microorganisms occur in a short time, and it does not synchronize with the slow movement of the pollutant in the region, resulting in low utilization efficiency and short effective period of the biodegradation. The nutrient loss into the ground water might need sufficient consideration since it may cause second environmental pollution.
No comprehensive means to solve such problems have been disclosed so far.
According to conventional methods to supply nutrients into the ground, the nutrients are dissolved in water, and the aqueous solution is directly sparged over the contaminated soil, or sent with pressure through a pipe provided in the ground. For example, in DE 3621313 C2 (1986. 6. 25 Chemic Fabrik, Dr. Worbs KG) is described a method where inorganic fertilizers (Na.sup.+, K.sup.+ and the like, ions of nitrogen, sulfur, and phosphorus, B, Mn, and Cu and the like) are repeatedly applied to the contaminated soil from the surface, and hydrogen peroxide and nitrate are applied into the soil as an inorganic oxidizing agent, or air (oxygen) is blown into the soil, to activate the soil microorganisms for pollutant degradation. In this method, it is difficult to supply an appropriate amount of the nutrients corresponding to the degradation activity of the degrading microorganisms, and it is necessary to control the amount of supply by monitoring the activity of the microorganisms in the soil. According to this method, the nutrients might be shared by the non-degrading microorganisms so that the input efficiency of the functional materials might be lowered, or the surplus nutrients and the like might be released.
In U.S. Pat. No. 5,080,782 and DE 3937593 C1, a pollutant-degrading method has been proposed in which ground water is pumped through a plurality of wells provided within the contaminated soil, and the ground water is treated in a bioreactor or a water storing tank, and then re-injected into the contaminated soil. This process allows accurate monitoring of the condition of the contaminated ground water, and the microbial activity function can be appropriately controlled in a bioreactor, on the other hand a sophisticated facility must be provided on the ground and the continuous control is required, thus spoils the advantages of the in situ bioremediation which utilizes the natural restoration power.
In U.S. Pat. No. 4,401,569 is disclosed a process in which a gas and nutrients are supplied to a region contaminated with halogenated hydrocarbons, the ground water is removed from the region through a draining trench, and the pollutant-degrading microbes in the soil are activated. Like the above-mentioned conventional methods, nutrients and the gas are given as fluid, a large amount of which is circulated in this method. Therefore, also this process has above-mentioned drawbacks that it requires a considerable size of equipment on the ground and that utilization efficiency of the supplied material is low since the nutrients supplied into wide region allows the proliferation of non-degrading microorganisms.
The supply of oxygen and nutrients to the microorganisms in the soil by the movement of natural air and water has similarity to the supply of fertilizers in agriculture. In agriculture, water soluble nutrients such as chemical fertilizers can be directly given to plants, while some organic fertilizers such as manure are once metabolized by microorganisms in soil and the resulting metabolites are utilized gradually by the plants. The former is quick acting, while the latter is relatively slow acting because of the mediation of microorganisms. In this context, there have been devised slow acting chemical fertilizers where release the nutrients slowly. In U.S. Pat. No. 4,657,576 to Johannes Lambie et al. is disclosed a slow-releasing chemical fertilizer consisting of urea dispersed in a wax. In U.S. Pat. No. 3,300,293 and U.S. Pat. No. 3,252,786 to Andrew Bozzelli et al. is proposed a chemical fertilizer having epoxy-polyester resin coating. Such a device applied to microbial remediation to make the component which is quick acting in fluid slow acting is disclosed in W091/19039 (1990 6. 8, Grace Sila Horticultural Products Company), in which a water soluble nutrient is encapsulated with linseed oil and dichloropentadiene resin to apply them into the contaminated soil. An aim of this proposal is the maintained pollutant degradation by the microorganisms.
In this process, the durability of the supply of the nutrients has been improved, but it has the same defect as that of giving the nutrient in the fluid form on a point that the nutrients are shared by ambient non-degrading microorganisms which do not contribute to the degradation of the pollutants.
There has been made a proposal from a different stand point, in which immobilized microorganisms and nutrients is located in a pass of polluted fluid.
In U.S. Pat. No. 4,810,385 discloses a process for treating waste water comprising disposing in a pass of waste water a vessel of a porous package in which dried microbial culture is sealed.